Bison no longer remembers the path to `m4` as of
Homebrew/homebrew-core#84931. Since superenv does not put runtime
dependencies of build dependences in `PATH`, we now need to help Bison
find `m4` by setting `M4` in the environment.
See also Homebrew/homebrew-core#85260.
- This also required auto-fixes for Layout/EmptyLinesAroundBlockBody and
Layout/InconsistentIndentation once the auto-fixer had got rid of the
"redundant begin"s.
`-Os` produces sometimes bigger binaries on Linux.
Also, llvm built with `-Os` is really slow at runtime for Linux.
Using `-02` aligns us with what Debian does, and as we are compiling most of our stuff with gcc (and not clang), it makes sense to use `-02` on Linux.
`-Os` does probably slightly different things when used on mac with llvm, compared to when it is used with gcc on Linux.
- atomic_write: close file before renaming to prevent error:
'Device or resource busy'
- ensure_writable: preserve executable bit
- new elf? and dynamic? methods
